1. Field of the Invention
The present invention relates to a steel material and a method of manufacturing the same, wherein the yield ratio of the steel material is controlled, whereby the steel material having the same tensile strength may easily undergo metal working such as plastic working, cutting, etc., and whereby the manufacturing cost thereof may also be decreased.
2. Description of the Related Art
In the manufacture of parts using steel-forged products, many attempts have been made to reduce manufacturing costs through process improvements and automation, among which the elimination of quenching and tempering (which is hereinafter referred to as “Q/T”) after forging is essential in order to save energy and improve process efficiency (in automation lines).
FIG. 1 is a flowchart illustrating the conventional process of manufacturing typical quenched and tempered steel using Q/T after forging.
As illustrated in FIG. 1, the quenched and tempered steel is manufactured in such a manner that spheroidizing is performed for 10 hr or longer to reduce hardness and forging load, cold forging is conducted, and then post heat treatment, namely, Q/T, is essentially carried out to ensure mechanical properties after the forging.
Due to pre/post heat treatment and the large number of processes involved, the manufacturing cost is excessively increased, and a correction process is additionally required owing to dimensional changes attributable to heat treatment deformation, undesirably negating the energy saving and automation benefits.
Hence, thorough research is ongoing into the development of a forging process and Non-Heat-Treated steel without the need for Q/T after the forging. Domestically, hot forged products are mainly applied to parts that do not require toughness, such as crankshafts, wheel hubs, etc., and Non-Heat-Treated steel products for hot forging are being developed and employed in knuckles and connecting rods for vehicles.
Also, Non-Heat-Treated steel for cold forging has been developed, and the application thereof has been attempted, but some examples (LH85, etc.) thereof are limitedly applied to bolts. Although this steel is advantageous because it obviates the need for Q/T after forging, the forging load may be excessively raised at a tensile strength of 80 kgf/mm2 or more, attributable to the high yield ratio thereof, undesirably shortening the lifetime of a mold, thereby making it impossible to broaden the scope of application thereof.
Furthermore, steel wires resulting from the continuous production of cold Non-Heat-Treated steel comprising a two-phase structure of ferrite and pearlite are problematic in terms of non-uniform quality because there is a significant material difference of 20% or more between the head portion and the tail portion. In order to solve this problem, as illustrated in FIG. 2, isothermal transformation has to be additionally carried out before cold forging, which may increase material costs, making it difficult to apply.